Method for joining oxide superconductors and joined oxide superconductor

ABSTRACT

The present invention provides a method for joining an RE123 oxide superconductor matrix obtained by a melt process by the use of a solder material. The (110) plane of an RE123 oxide superconductor matrix obtained by a melt process is used as the plane to be joined, a solder material composed of an RE123 oxide superconductor having a lower melting point than the above-mentioned RE123 oxide superconductor is interposed between the planes to be joined, and this solder material is melted and then solidified to form a joining layer, thereby joining the matrices. The solder material can be a sinter, a melt-processed material, a powder, a slurry, or a molded powder.

FIELD OF TECHNOLOGY

The present invention relates to a method for joining oxidesuperconductors, and to a joined oxide superconductor that has beenjoined by this method.

BACKGROUND TECHNOLOGY

Superconducting oxide materials with a high superconducting transitiontemperature (T_(c)), such as LiTi₂O₃, Ba(Bi, Pb)O₃, (Ba, K)BiO₃, (La,Sr)₂CuO₄, REBa₂Cu₃O_(7−δ) (RE is a rare earth element), Bi₂Sr₂Ca₂Cu₃O₁₀,Ti₂Ba₂Ca₂Cu₃O₁₀, or HgBa₂Ca₂Cu₃O₈, have been discovered one afteranother in recent years. Superconductors composed of these materials areable to generate a powerful electromagnetic force through interactionwith a magnetic field, and their practical application in various fieldsin which this force is utilized, such as bearings, flywheels and loadtransport system has therefore been studied.

Of these superconducting oxide materials, those based on REBa₂Cu₃O_(7−δ)in particular (hereinafter referred to as “RE123 oxide superconductingmaterials”; the RE here is one or more members of the group consistingof Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) have a highcritical temperature. In addition, they have a high critical currentdensity in a magnetic field due to development and improvement in theirmanufacturing technology and recently have become one of the mostnoteworthy superconducting materials.

It has also become clear that superconductors with such a large criticalcurrent can function as permanent magnets by trapping a strong magneticfield or, conversely, can shield a strong magnetic field, so in additionto the applications mentioned above, applications such as magneticshields and permanent magnets are also on the horizon.

The most common method to produce an oxide superconductor (bulk) is a“melt solidification process,” in which a molten oxide superconductingmaterial (crystal precursor) is solidified while being slowly cooledfrom near its solidification temperature, the result being the growth ofcrystals. Another manufacturing method is the “supercooling meltsolidification process,” in which the crystal growth time is shortened.This method involves supercooling a molten crystal precursor down to atemperature below the solidification temperature while the precursor isstill in a molten or semimolten state, then slowly cooling from thistemperature or maintaining this temperature to grow crystals. The goalhere is to raise the crystal growth rate through supercooling (JapanesePatent Publication H6-211588).

Still, the surface area of a superconducting oxide material needs to beincreased if the superconductor is to be used as a magnetic shield orother such material as mentioned above. Furthermore, oxidesuperconductors have anisotropy in materials properties depending ontheir crystal orientation, with the current flowing mainly in the a-bdirection of the crystals, so for them to be used as a magnetic shield,the sample should be installed so that the c axis is perpendicular tothe magnetic field.

However, at the present time the bulk superconductors obtained by theabove method are only a few centimeters at the largest size, and it isextremely difficult to produce larger superconductors.

-   -   This means that a large superconductor must be produced by        joining small superconductor blocks.

Some of the joining methods known in the past are introduced below.

(1) K. Salama and V. Selvamanickem (Appl. Phys. Lett. 60 (1992), 898)

-   -   Samples are joined, without any deterioration of superconducting        properties at the joined interface, by heating YBa₂Cu₃O_(7−δ)        superconductors (Y123), which were produced by melt process, for        approximately 30 hours under uniaxial pressure of 2 to 6 MPa at        a temperature between 910 and 930° C., without interposing        solder material between the joined interfaces.

(2) “Advances in Superconductivity VII,” Springer-Verlag, Tokyo, 1995,pp. 681–684

-   -   A matrix of a Y123 superconducting bulk        (Y_(1.8)Ba_(2.4)Cu_(3.4)O_(y)) is prepared. A powder solder of a        Yb123 superconducting material (Y_(1.2)Ba_(2.1)Cu_(3.1)O_(y))        produced by melt solidification process and having a lower        melting point (peritectic point) than the matrix is sandwiched        between said matrices. The temperature is raised to between the        melting point of the matrix and that of the solder to bring the        solder into a semimolten state. This is then gradually cooled to        epitaxially grow crystals of the solder material (Yb123        crystals) from the matrix surface, joining the matrices together        via this crystallized solder.

(3) Japanese Patent Publication H7-82049

-   -   A component that will enter the liquid phase at the joining        temperature or a component that readily undergoes        high-temperature creep, such as a composition based on Ag,        BaCuO₂—CuO or REBa₂Cu₃O_(7−δ) (RE=Y, Ho, Er, Tm, Yb) is        interposed by coating, vapor deposition, or another such method        at the joined interface between matrices of a yttrium-based        oxide superconductor produced by the melt solidification        process. After that, this sample is heated under pressure for 1        to 10 hours at a temperature of 900 to 990° C. to fuse it, and        then the sample is cooled at a rate of 2° C./hour or less to        join the matrices together.

If we use the above joining method (2) as an example of a conventionaljoining technique, the following problems are encountered.

-   -   Specifically, when a solder is sandwiched between matrices and        then heated and slowly cooled, the Yb123 crystallization of the        semimolten solder gradually proceeds from the Y123 matrix        surface toward the center of the solder. So, the final        solidified portion of the solder forms unreacted        non-superconductive layer at the middle location of the        thickness of the solder sandwiched between the matrices.

The solder (crystal precursor) that has been heated to a hightemperature and becomes a semimolten liquid phase includes anon-superconducting BaO—CuO melt and a non-superconducting Yb211 phase.This Yb211 phase reacts with the melt, forming superconducting Yb123crystals while solidifying, however, a mixture of the above-mentionednon-superconducting portion such as Yb211 phase and BaO—CuO tends toremain in a layer form in the final solidified portion. Also, the solderthat has become a semimolten liquid phase contains numerous voids,impurities, and so on, and these also tend to remain as a layer in thefinal solidified portion.

This is in part due to the so-called “pushing” effect, in which theYb211 phase, BaO—CuO, impurities, voids and so forth is pushed forwardthe unsolidified middle portion of the solder. That is caused by theepitaxial growth of Yb123 crystals from the matrix surface toward themiddle of the solder during gradual cooling after heating.

FIG. 9 consists of schematic diagrams illustrating this “pushing”effect. FIG. 10 is a schematic diagram illustrating a joint at whichpores and segregation products are present as a result of this pushing.

For example, a solder of a Yb123 superconducting material composition issandwiched between Y123 superconducting bulk matrices and heated untilthe solder becomes semimolten, as shown in FIG. 9( a). This semimoltensolder includes a Yb211 phase, bubbles, and so forth, which arecontained in the molten BaO—CuO. As these slowly cool, as shown in FIG.9 b, the Yb211 phase reacts with the BaO—CuO melt, which producessuperconducting Yb123 crystals that grow from the Y123 matrix surfacetoward the center of the solder. The Yb211, bubbles, and so forthpresent in the unsolidified melt here are pushed away from the growthfront of the Yb123 crystal and concentrate in the middle part of theunsolidified solder. As the Yb123 crystals continue to grow and thesolder reaches its final solidification stage, as shown in FIG. 9( c),the unreacted Yb211 phase, bubbles, and so forth segregate increasinglytowards the center of the solder. The un-reacted Yb211 phase, bubbles,and so forth finally segregate in layer form over the entire crosssection of the middle part of the solder, in which state thesolidification of the solder is completed, joining the Y123 matrices.

Since the Yb211 phase, bubbles, and so forth are not superconducting,the superconducting characteristics of a joined oxide superconductor bythe above method, especially in this joined portion, are markedlydegraded.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a solution to thedrawbacks encountered with the conventional joining methods, and morespecifically it is an object to provide a method for joining an RE123oxide superconductor that will result in no segregation of impuritiesand pores in the joined portion.

It is a further object of the present invention to provide a stronglycoupled RE123 oxide superconductor obtained by the above joining method.

As a result of diligent research aimed at solving the above problems,the inventors learned the following about the joining of RE123superconductor matrices.

(a) A good joint will be obtained if the plane to be joined of thematrix is parallel to the (110) crystallographic plane.

(b) In the case of (a) above, a better joint will be obtained if a highdensity RE123 superconductor compact such as a sinter and melt-processedplate that has a lower melting point than the matrix is selected as thesolder material and if the material is heated, melted and solidified.

The inventors perfected the present invention on the basis of the abovefindings, and aspects of the present invention are as follows.

-   -   (1) A method for joining an RE123 oxide superconductor (RE: one        or more members of the group consisting of Y, La, Pr, Nd, Sm,        Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) produced by a melt process,        comprising:    -   aligning a plane to be joined of an RE123 oxide I superconductor        so as to be parallel to the (110) crystallographic plane;    -   interposing a solder material composed of an RE123 oxide        superconductor material (RE: one or more members of the group        consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and        Lu) having a lower melting point than the RE123 oxide        superconductor between the planes to be joined; and melting and        then solidifying said solder material to form a joining layer.

(2) The method of (1) above, wherein the solder material is a sinter ora melt-processed plate.

(3) The method of (2) above, wherein a surface of the solder material isfinely polished.

(4) The method of (1), wherein the solder material is a powder, aslurry, or a molded powder.

(5) The method of any of (1) to (4) above, wherein the RE123 oxidesuperconductor contains a non-superconducting phase including RE.

(6) The method of claim 5, wherein the non-superconducting phase is anRE₂BaCuO₅ phase (RE211 phase) and/or an RE_(4−x)Ba_(2+x)Cu₂O_(10−y)phase (RE422 phase, 0≦x≦0.2, 0≦y≦0.5).

(7) The method of (5) above, wherein the non-superconducting phase is anRE₂BaCuO₅ phase (RE211 phase) and/or anRE_(4−2x)Ba_(2+2x)Cu_(2−x)O_(10−y) phase (RE422 phase, 0≦x≦0.3,0≦y≦0.6).

(8) The method of any of (1) to (7) above, wherein the RE123 oxidesuperconductor contains one or more members of the group consisting ofAg, Pt, CeO₂, and Ag₂O.

(9) The method of any of (1) to (8) above, wherein the solder materialcontains one or more members of the group consisting of Ag, Pt, CeO₂,and Ag₂O.

(10) The method of any of (1) to (9) above, wherein a pressure isapplied during joining.

(11) A joined RE123 oxide superconductor, in which the (110) planes of aplurality of RE123 oxide superconductors (RE: one or more members of thegroup consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, andLu) produced by a melt process have been joined with a solder materialcomposed of an RE123 oxide superconductor material (RE: one or moremembers of the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho,Er, Tm, Yb, and Lu) having a lower melting point than the RE123 oxidesuperconductor to be joined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the method employed in a sample joiningtest;

FIG. 2 is a diagram illustrating the shape of the sample for measuringthe joined sample;

FIG. 3 consists of compositional images and characteristic X-ray imagesnear the joined interface of the joined product of the present inventionusing the (110) plane to be joined;

FIG. 4 is a graph of the temperature dependence of the magnetization ofthe joined product of the present invention;

FIG. 5 is a graph of the external magnetic field dependence of thecritical current density of the joined product of the present invention;

FIG. 6 consists of magneto-optical micrographs of a sample in which thejoined plane of the joined product of the present invention is parallelto the (110);

FIG. 7 consists of compositional images and characteristic X-ray imagesnear the interface of the joined product of a comparative example inwhich the joined plane is parallel to the (100) plane;

FIG. 8 consists of magneto-optical micrographs of the joined product ofa comparative example in which the joined plane is parallel to the (100)plane;

FIG. 9 consists of diagrams to explain the pushing phenomenon seen witha conventional Y123 superconductor bulk joining method;

FIG. 10 is a diagram illustrating the pores and segregation ofimpurities observed at the joint obtained with a conventional Y123superconductor bulk joining method; and

FIG. 11 consists of magneto-optical micrographs of the joined product ofthe present invention in which the joined plane is parallel to the (110)plane.

BEST MODE OF CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detail.

(Crystal Orientation of the Matrix Plane to be Joined)

In the past, the (100) plane has been employed as the orientation of theplane to be joined in the joining of RE123 superconductor matrices.

In contrast, tests conducted by the inventors into the orientation ofthe plane to be joined have revealed that better joining is possible ifthe orientation of the face is parallel to the (110) crystallographicplane.

Therefore, in the present invention, the (110) plane is used as theRE123 superconductor plane to be joined. While it is preferable for thejoined plane to be parallel to the (110) plane, it does not have to beparallel the (110) in the strict sense. Deviation from this orientationup to about 15° is permissible, although 7° or less is preferred. Thephrase “the plane to be joined of the RE123 oxide superconductor isparallel to the (110) plane” as used in this Specification encompassesthe abovementioned case in which the deviation is within 15°.

(Superconductor Matrix)

The superconductor matrix to which the method of this invention isapplied is an RE123 oxide superconductor (where RE is one or moremembers of the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho,Er, Tm, Yb, and Lu), and is expressed by the general formulaREBa₂Cu₃O_(7−δ). In order to increase the critical current of theabove-mentioned RE123 oxide superconductor, a non-superconducting phaseincluding RE may be dispersed in the matrix phase of the RE123 oxidesuperconductor. Examples of this non-superconducting phase include anRE₂BaCuO₅ phase (RE211) and an RE_(4−x)Ba_(2+x)Cu₂O_(10−y) phase (RE422phase, 0<x<0.2, 0<y<0.5) (see Japanese Patent Publication 2,828,396).

This RE_(4−x)Ba_(2+x)Cu₂O_(10−y) phase can also be expressed as anRE_(4−2x)Ba_(2+2x)Cu_(2−x)O_(10−y) phase, and both forms of notationwill be used in the present invention.

The RE123 oxide superconductor of the matrix may contain one or moremembers of the group consisting of Ag, Pt, CeO₂, and Ag₂O. In this casethe platinum and CeO₂ have the effect of finely dispersing RE211 andRE422 in the RE123, while the silver and Ag₂O have the effect ofincreasing the mechanical strength of a composite (bulk) of RE123+RE211or RE422.

(Solder Material)

An RE123 oxide superconductor (where RE is one or more members of thegroup consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, andLu) having a lower melting point than the matrix is used as the soldermaterial.

Since the melting point of an RE123 oxide superconductor varies with theRE used, the RE of the solder material is appropriately selectedaccording to the type of matrix being used so that the melting point ofthe solder material will be lower than the melting point of the matrix.

The solder material, just as with the matrix, may contain an RE221 andan RE422 as a dispersion phase, and one or more members of the groupconsisting of Ag, Pt, CeO₂, and Ag₂O may also be contained for the samereasons as those given for the matrix.

The following is a favorable example of a material that can be used forthe solder material.(1−x) REBa₂Cu₃O_(7−δ) +xRE₂BaCuO₅ +y mass % A

-   -   Where    -   RE is one or more members of the group consisting of Y, La, Pr,        Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu,    -   y is the proportion (mass %) against (1−x)        REBa₂Cu₃O_(7−δ)+xRE₂BaCuO₅ defined as 100 mass %, where 0≦y≦40,        and    -   A is one or more members of the group consisting of Ag, Pt,        CeO₂, and Ag₂O.

The solder material can be used in the form of either a powder, aslurry, a molded powder, a sinter, or a melt-processed plate, but asinter or a melt-processed plate is preferable.

The present invention is characterized in that the face of the matrix tobe joined is parallel to the (110) plane. Even in this case, using asinter or a melt-processed plate as the solder material will yield abetter joint than when a powder, slurry, or molded powder is used. Thereason for this is following. When a powder, slurry, or molded powder isused as the solder material, the air present between the powderparticles cannot escape even in the course of the melting andsolidification of the solder, and therefore forms bubbles that remain inthe joint. Whereas, when the solder material is a sinter or amelt-processed plate, since the solder material is closely packed, thereis less air between the particles, so air is less apt to remain in thejoint. When the solder material is a sinter or a melt-processed plate,since solder material is present at a higher density during heating andmelting, the melt phase of the solder material more readily undergoesepitaxial growth on the matrix crystal surface in the recrystallizationprocess.

In the joining method of the present invention, the solder material ismelted and then solidified to form the joining layer. In this case, theterm “melted” as used in reference to the solder material in thisSpecification also encompasses a semimolten state in whichrecrystallization is possible.

EXAMPLES

Examples of the present invention will now be given along withcomparative examples, but the present invention is not limited to or bythese examples.

Example 1 Joining a Y—Ba—Cu—O Bulk with an Er—Ba—Cu—O Solder Material

Preparation of Matrix

A yttrium-based superconducting oxide material bulk (single domain withc-axis orientation; QMG made by Nippon Steel Corporation) was cut into a3×4×5 mm³ (a plane to be joined was 4×5 mm²) rectangular parallelepipedsuch that the plane to be joined was parallel to the (110) plane. Thesurface of this cut sample to be joined was polished to a mirror finishto produce a matrix to be joined (hereinafter referred to as “matrixA”).

Preparation of Solder Material

Raw powders of Er₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.75ErBa₂Cu₃O_(7−δ)+0.25Er₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined for 24 hoursat 890° C. in a pure oxygen atmosphere. This calcination was repeateduntil all of the raw powder became two phases of ErBa₂Cu₃O_(7−δ) andEr₂BaCuO₅.

Platinum was added to the mixed powder having two phases in an amount of0.5 mass % with respect to the total mass of the mixed powder, and thiswas mixed for another 3 hours in an automated mortar and pestle. Afterthis mixing, the mixture was molded into a rectangular parallelepipedapproximately 18×9×5 mm³ in size, and then pressed in a cold isostaticpress (hereinafter referred to as CIP).

This molding was sintered by heating for 10 hours under atmosphericconditions and at 975° C., which is 10° C. lower than 985° C., which isthe peritectic reaction temperature of the ErBa₂Cu₃O_(7−δ).

The sinter thus obtained was cut into a plate with a thickness ofapproximately 1 mm, and further polished to produce a spacer with athickness of 0.5 mm. The spacer was polished until both sides had amirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1.

-   -   The sample was heated for 3 hours to 995° C., held at this        temperature for 1 hour, and then cooled to 945° C. at a rate of        0.5° C. per hour.        Evaluation

After the heat treatment, a sample to be measured was cut from theobtained joined product so as to include the joined interface as shownin FIG. 2, part of this was polished in order to observe themicrostructure at the joined interface, and the other part was subjectedto oxygen annealing for 150 hours at 520° C. in an oxygen atmosphere inorder to evaluate the superconductivity.

FIG. 3 consists of composition images and characteristic X-ray images(Ba-Mα line and Cu-Kα line) near the joined interface. It can be seenthat the joined portion is extremely dense, with no pores visible,indicating that the joining process is successful. Furthermore, it canbe seen that there is no segregation of CuO or the like, indicating thatthe microstructure is extremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with asuperconducting quantum interference device (SQUID) magnetometer. Amagnetic field was applied parallel to the c axis of the sample beingmeasured, and an external magnetic field of 10 Oe was applied in themeasurement of the superconductivity transition temperature. Thetemperature was 77 K in the measurement of the magnetic field dependenceof the critical current density. The results are shown in FIGS. 4 and 5.The superconductivity transition temperature was 92 K. The criticalcurrent density was approximately 13,000 A/cm² at an external magneticfield of 2 T. The irreversible magnetic field was approximately 4.8 T.

Further, magneto-optical effect (MO) was utilized to measure themagnetic field distribution in order to observe the effect of theexternal magnetic field on the joint properties. A magnetic field wasapplied parallel to the c axis of the sample. The results are shown inFIG. 6. The experiment temperature was 77 K. Magnetic flux penetratedinto only part of the joint even when an external magnetic field of over1000 Oe was applied. Penetration of the magnetic flux into the joint wasobserved at 1400 Oe.

Comparative Example 1

Other than cutting the matrix such that the plane to be joined of thematrix would be parallel to the (100) plane, a joined product wasproduced in the same manner as in Example 1, and a sample to beevaluated was produced from this joined product by the same procedure asin Example 1.

FIG. 7 consists of composition images and characteristic X-ray images(Ba-Mα line and Cu-Kα line) near the joined interface. It can be seenthat the joined portion is extremely dense, with no pores visible,indicating that the joining process is successful. However, segregationof CuO was noted in the middle part of the joint.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. The superconductivity transition temperature of the samplewas 92 K. The critical current density was approximately 5000 A/cm² atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 4.0 T.

Magneto-optical effect (MO) was then utilized to measure the magneticfield distribution. The results are shown in FIG. 8. The experimenttemperature was 77 K. Magnetic flux penetrated completely into the jointwhen an external magnetic field of about 200 Oe was applied.

Example 2 Joining a Y—Ba—Cu—O Bulk with a Yb—Ba—Cu—O Solder Material

Preparation of Matrix

The matrix was prepared in the same manner as matrix A in Example 1.

Preparation of Solder Material

Raw powders of Yb₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.75YbBa₂Cu₃O_(7−δ)+0.25Yb₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined three timesfor 24 hours each time at 890° C., two times for 50 hours at 920° C.,and one time for 100 hours at 920° C. in a pure oxygen atmosphere. Thiscalcination was repeated until all of the raw powder became two phasesof YbBa₂Cu₃O_(7−δ) and Yb₂BaCuO₅.

Platinum was added to the mixed powder having two phases in an amount of0.5 mass % with respect to the total mass of the mixed powder, and thiswas mixed for another 3 hours in an automated mortar and pastle. Afterthis mixing, the mixture was molded into a rectangular parallelepipedapproximately 18×9×5 mm³ in size, and then pressed in a CIP.

This molding was sintered by heating for 10 hours under atmosphericconditions and at 942° C., which is 10° C. lower than the 952° C.peritectic reaction temperature of the YbBa₂Cu₃O_(7−δ).

The sinter thus obtained was cut into a plate with a thickness ofapproximately 1 mm, and further polished to produce a spacer with athickness of 0.5 mm. The spacer was polished until both sides had amirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1.

-   -   The sample was heated for 3 hours to 995° C., held at this        temperature for 1 hour, and then cooled to 945° C. at a rate of        0.5° C. per hour.        Evaluation

After the heat treatment, a sample to be measured was produced from theobtained joined product by the same procedure as in Example 1.

Composition images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed that the joined portion wasextremely dense, with no pores visible, indicating that the joiningprocess was successful. Furthermore, it can be seen that there was nosegregation of CuO or the like, indicating that the microstructure wasextremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer, revealing the superconductivity transition temperature tobe 92 K. The critical current density was approximately 10,000 A/cm² atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 4.0 T.

Next, magneto-optical effect (MO) was utilized to measure the magneticfield distribution. The experiment temperature was 77 K. Magnetic fluxpenetrated into only part of the joint even when an external magneticfield of over 500 Oe was applied.

Comparative Example 2

Other than cutting the matrix such that the plane of the matrix to bejoined would be parallel to the (100) plane, a joined product wasproduced in the same manner as in Example 3, and an evaluation samplewas produced from this joined product by the same procedure as inExample 3.

Composition images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed that the joined portion wasextremely dense, with no pores visible, indicating that the joiningprocess was successful. However, segregation of CuO was noted in themiddle part of the joint.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. The superconductivity transition temperature of the samplewas 92 K. The critical current density was approximately 5000 A/cm² atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 3.7 T.

Magneto-optical effect (MO) was then utilized to measure the magneticfield distribution. The experiment temperature was 77 K. Magnetic fluxpenetrated completely into the joint when an external magnetic field ofabout 400 Oe was applied.

Example 3 Joining a Y—Ba—Cu—O Bulk with a Y—Ba—Cu—O Solder Material

Preparation of Matrix

The matrix was prepared in the same manner as matrix A in Example 1

Preparation of Solder Material

Raw powders of Y₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.75YBa₂Cu₃O_(7−δ)+0.25Y₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined for 24 hoursat 890° C. in a pure oxygen atmosphere. This calcination was repeateduntil all of the raw powder became two phases of YBa₂Cu₃O_(7−δ) andY₂BaCuO₅.

Platinum and Ag₂O were added to the mixed powder having two phases inamounts of 0.5 mass % and 10 mass %, respectively, with respect to thetotal mass of the mixed powder, and this was mixed for another 3 hoursin an automated mortar and pestle. After this mixing, the mixture wasmolded into a rectangular parallelepiped approximately 18×9×5 mm³ insize, and then pressed in a CIP. This molding was sintered by heatingfor 10 hours under atmospheric conditions and at 960° C., which is 10°C.lower than the 970° C. peritectic reaction temperature of the0.75YBa₂Cu₃O_(7−δ)+0.25Y₂BaCuO₅+0.5 mass % Pt+10 mass % Ag₂O.

The sinter thus obtained was cut into a plate with a thickness ofapproximately 1 mm, and further polished to produce a spacer with athickness of 0.5 mm. The spacer was polished until both sides had amirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1. The sample was heated for 3 hours to 950° C.,held at this temperature for 1 hour, and then cooled to 900° C. at arate of 0.5° C. per hour.

Evaluation

After the heat treatment, a sample to be measured was produced from theobtained joined product by the same procedure as in Example 1.

Composition images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed that the joined portion wasextremely dense, with no pores visible, indicating that the joiningprocess was succesful. Furthermore, it can be seen that there was nosegregation of CuO or the like, indicating that the microstructure wasextremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer, revealing the superconductivity transition temperature tobe 91 K. The critical current density was approximately 12,000 A/cm2 atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 5.0 T.

Example 4 Joining an Sm—Ba—Cu—O Bulk with a Gd—Ba—Cu—O Solder Material

Preparation of Matrix

Raw powders of Sm₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.75SmBa₂Cu₃O_(7−δ)+0.25Sm₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined for 24 hoursat 880° C. under atmospheric conditions. After this calcination, thesample was pulverized again and then calcined three more times by thesame method as above. Next, the calcined powder was sintered for 24hours at 980° C. in an atmosphere of 1% O₂+99% Ar. The sintering andpulverization were repeated until all of the raw powder became twophases of SmBa₂Cu₃O_(7−δ) and Sm₂BaCuO₅.

Platinum was added to the mixed powder having two phases in an amount of0.5 mass % with respect to the total mass of the mixed powder, and thiswas mixed for another 3 hours in an automated mortar and pestle. Afterthis mixing, the mixture was molded into a cylinder with a diameter of30 mm and a height of 12 mm, and then pressed in a CIP.

This molding was placed on a rod of yttrium-stabilized ZrO₂ and set in atube furnace with a controllable firing atmosphere. The sample washeated for 3 hours to 1150° C. in an atmosphere of 1% O₂+99% Ar, andafter being held at this temperature for 1 hour was immediately cooledto 1020° C. over a period of 15 minutes. NdBa₂Cu₃O_(7−δ) seed crystal,which had been produced in advance by melt solidification process, wasplaced on the sample during this cooling. After this, the sample wascooled down to 960° C. at a rate of 0.75° C. per hour. The sample thusproduced was in the form of single domain with a uniform crystalorientation.

The samarium-based superconducting oxide material bulk obtained abovewas cut into a 3×4×5 mm³ (a plane to be joined was 4×5 mm²) rectangularparallelepiped such that the plane to be joined was parallel to the(110) plane. The joining surface of this cut sample was polished to amirror finish.

Preparation of Solder Material

Raw powders of Gd₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.75GdBa₂Cu₃O_(7−δ)+0.25Gd₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined for 24 hoursat 880° C. under atmospheric conditions. After the calcination, thesample was pulverized again and then calcined three more times by thesame method as above. Next, the calcined powder was sintered for 24hours at 965° C. in an atmosphere of 1% O₂+99% Ar. The sintering andpulverization were repeated until all of the raw powder became twophases of GdBa₂Cu₃O_(7−δ) and Gd₂BaCuO₅.

Platinum was added to the mixed powder having two phases in an amount of0.5 mass % with respect to the total mass of the mixed powder, and thiswas mixed for another 3 hours in an automated mortar and pestle. Afterthis mixing, the mixture was molded into a rectangular parallelepipedapproximately 18×9×5 mm³ in size, and then pressed in a CIP.

This molding was sintered by heating for 10 hours in an atmosphere of 1%O₂+99% Ar and at 975° C., which is 10° C. lower than the 985° C.peritectic reaction temperature of the GdBa₂Cu₃O_(7−δ).

The sinter thus obtained was cut into a plate with a thickness ofapproximately 1 mm, and further polished to produce a spacer with athickness of 0.5 mm. The spacer was polished until both sides had amirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1. The sample was heated for 3 hours to 1000° C.in an atmosphere of 1% O₂+99% Ar, held at this temperature for 1 hour,and then cooled to 950° C. at a rate of 0.5° C. per hour.

Evaluation

After the heat treatment, a sample was cut from the obtained joinedproduct so as to include the joined interface as shown in FIG. 2, partof this was polished in order to observe the microstructure at thejoined interface, and the other part was subjected to oxygen annealing,in which the temperature was raised to 450° C. over 3 hours in an oxygenatmosphere, then the temperature was lowered to 300° C. over 50 hours,and finally the temperature was held there for 150 hours, in order toevaluate the superconductivity characteristics.

Texture images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed that the joined portion wasextremely dense, with no pores visible, indicating that the joiningprocess was successful. Furthermore, it can be seen that there was nosegregation of CuO or the like, indicating that the microstructure wasextremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. The superconductivity transition temperature of the samplewas 94 K. The critical current density was approximately 20,000 A/cm² atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 6.0 T.

Example 5 Joining an Sm—Ba—Cu—O/Ag Bulk with a Gd—Ba—Cu—O/Ag SolderMaterial

Preparation of Matrix

A silver-added samarium-based superconducting oxide material bulk(single domain with c-axis orientation; made by Nippon SteelCorporation) was cut into a 3×4×5 mm³ (a plane to be joined was 4×5 mm²)rectangular parallelepiped such that the plane to be joined was parallelto the (110) plane. The joining surface of this cut sample was polishedto a mirror finish to produce a matrix to be joined.

Preparation of Solder Material

Commercially available raw powders of GdBa₂Cu₃O_(7−δ) and Gd₂BaCuO₅ wereweighed out such that the composition would be0.75GdBa₂Cu₃O_(7−δ)+0.25Gd₂BaCuO₅, and these were mixed for 3 hours inan automated mortar and pestle. Pt and Ag₂O were added to the mixedpowder in amounts of 0.5 mass % and 10 mass %, respectively, withrespect to the total mass of the mixed powder, and this was mixed foranother 3 hours in an automated mortar and pestle. After this mixing,the mixture was molded into a rectangular parallelepiped approximately18×9×5 mm³ in size, and then pressed in a CIP.

This molding was sintered by heating under atmospheric conditions and at1000° C., which is 10° C. lower than the 1010° C. peritectic reactiontemperature of the GdBa₂Cu₃O_(7−δ)+Ag₂O. The sinter thus obtained wascut into a plate with a thickness of approximately 1 mm, and furtherpolished to produce a spacer with a thickness of 0.5 mm. The spacer waspolished until both sides had a mirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1. The sample was heated for 3 hours to 1023° C.under atmospheric conditions, held at this temperature for 1 hour, andthen cooled to 973° C. at a rate of 0.5° C. per hour.

Evaluation

After the heat treatment, a sample to be measured was cut from theobtained joined product so as to include the joined interface as shownin FIG. 2, part of this was polished in order to observe themicrostructure at the joined interface, and the other part was subjectedto oxygen annealing, in which the temperature was raised to 450° C. over3 hours in an oxygen atmosphere, then the temperature was lowered to300° C. over 50 hours, and finally the temperature was held there for150 hours, in order to evaluate the superconductivity characteristics.

Texture images and characteristic X-ray images (Ba-Mα line, Cu-Kα line,and Ag-Mα line) near the joined interface revealed that the joinedportion was extremely dense, with no pores visible, indicating that thejoining process was succesful. Furthermore, it can be seen that therewas no segregation of CuO, silver, or the like, indicating that themicrostructure was extremely uniform.

Next, magneto-optical effect (MO) was utilized to measure the magneticfield distribution. The results obtained are shown in FIG. 11. Theexperiment temperature was 77 K. No magnetic flux penetrated into thejoint even when an external magnetic field of 1000 Oe or more wasapplied. Furthermore, it was confirmed that the sample completelytrapped the magnetic field when an external magnetic field of 2000 Oewas applied and the external magnetic field was then removed.

Example 6 Joining an Nd—Ba—Cu—O Bulk with an Sm—Ba—Cu—O Solder Material

Preparation of Matrix

Raw powders of Nd₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.875NdBa₂Cu₃O_(7−δ)+0.125Nd₄Ba₂Cu₂O₁₀, and thesewere mixed for 3 hours in an automated mortar and pestle. The mixedpowder was molded in a uniaxial press, after which it was calcined for24 hours at 880° C. under atmospheric conditions. After thiscalcination, the sample was pulverized again and then calcined threemore times by the same method as above. Next, the calcined powder wassintered for 24 hours at 980° C. in an atmosphere of 1% O₂+99% Ar. Thesintering and pulverization were repeated until all of the raw powderbecame two phases of NdBa₂Cu₃O_(7−δ) and Nd₄Ba₂Cu₂O₁₀.

CeO2 was added to the mixed powder having two phases in an amount of 1.0mass % with respect to the total mass of the mixed powder, and this wasmixed for another 3 hours in an automated mortar and pestle. After thismixing, the mixture was molded into a cylinder with a diameter of 30 mmand a height of 12 mm, and then pressed in a CIP.

This molding was placed on a rod of yttrium-stabilized ZrO₂ and set in atube furnace with a controllable firing atmosphere. The sample washeated for 3 hours to 1150° C. in an atmosphere of 1% O₂+99% Ar, afterbeing held at this temperature for 1 hour, then the sample wasimmediately cooled to 1045° C. over a period of 15 minutes.NdBa₂Cu₃O_(7−δ) seed crystal, which had been produced in advance by meltsolidification process, was placed on the sample during this cooling.After this, the sample was cooled down to 970° C. at a rate of 0.75° C.per hour. The sample thus produced was in the form of single domain witha uniform crystal orientation.

The neodymium-based superconducting oxide material bulk obtained abovewas cut into a 3×4×5 mm³ (a plane to be joined was 4×5 mm²) rectangularparallelepiped such that the plane to be joined was parallel to the(110) plane. The surface of this cut sample to be joined was polished toa mirror finish.

Preparation of Solder Material

Raw powders of Sm₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.75SmBa₂Cu₃O_(7−δ)+0.25Sm₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined for 24 hoursat 880° C. under atmospheric conditions. After this calcination, thesample was pulverized again and then calcined three more times by thesame method as above. Next, the calcined powder was sintered for 24hours at 980° C. in an atmosphere of 1% O₂+99% Ar. The sintering andpulverization were repeated until all of the raw powder became twophases of SmBa₂Cu₃O_(7−δ) and Sm₂BaCuO₅.

Platinum was added to the mixed powder having two phases in an amount of0.5 mass % with respect to the total mass of the mixed powder, and thiswas mixed for another 3 hours in an automated mortar and pestle. Afterthis mixing, the mixture was molded into a rectangular parallelepipedapproximately 18×9×5 mm³ in size, and then pressed in a CIP.

This molding was sintered by heating for 10 hours in an atmosphere of 1%O₂+99% Ar and at 1008° C., which is 10° C. lower than the 1018° C.peritectic reaction temperature of the SmBa₂Cu₃O_(7−δ).

The sinter thus obtained was cut into a plate with a thickness ofapproximately 1 mm, and further polished to produce a spacer with athickness of 0.5 mm. The spacer was polished until both sides had amirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1. The sample was heated for 3 hours to 1030° C.in an atmosphere of 1% O₂+99% Ar, held at this temperature for 1 hour,and then cooled to 980° C. at a rate of 0.5° C. per hour.

Evaluation

After the heat treatment, a sample was cut from the obtained joinedproduct so as to include the joined interface as shown in FIG. 2, partof this was polished in order to observe the microstructure at thejoined interface, and the other part was subjected to oxygen annealing,in which the temperature was raised to 450° C. over 3 hours in an oxygenatmosphere, then the temperature was lowered to 300° C. over 50 hours,and finally the temperature was held there for 150 hours, in order toevaluate the superconductivity characteristics.

Texture images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed that the joined portion wasextremely dense, with no pores visible, indicating that the joiningprocess was successful. Furthermore, it can be seen that there was nosegregation of CuO or the like, indicating that the microstructure wasextremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. The superconductivity transition temperature of the samplewas 95 K. The critical current density was approximately 25,000 A/cm² atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 6.0 T.

Example 7 Joining an [Nd, Eu, Gd]—Ba—Cu—O Bulk with a Gd—Ba—Cu—O SolderMaterial

Preparation of Matrix

Raw powders of Nd₂O₃, Eu₂O₃, Gd₂O₃, BaCO₃, and CuO were weighed out suchthat the composition would be0.90(Nd_(0.33)Eu_(0.33)Gd_(0.33))Ba₂Cu₃O_(7−δ)+0.10(Nd_(0.33)Eu_(0.33)Gd_(0.33))₂BaCuO₅,and these were mixed for 3 hours in an automated mortar and pestle. Themixed powder was molded in a uniaxial press, after which it was calcinedfor 24 hours at 880° C. under atmospheric conditions. After thiscalcination, the sample was pulverized again and then calcined threemore times by the same method as above. The calcined powder was thensintered for 24 hours at 950° C. in an atmosphere of 0.1% O₂+99.9% Ar.The sintering and pulverization were repeated until all of the rawpowder became two phases of (Nd_(0.33)Eu_(0.33)Gd_(0.33))Ba₂Cu₃O_(7−δ)and (Nd_(0.33)Eu_(0.33)Gd_(0.33))₂BaCuO₅.

Platinum was added to the mixed powder having two phases in an amount of0.5 mass % with respect to the total mass of the mixed powder, and thiswas mixed for another 3 hours in an automated mortar and pestle. Afterthis mixing, the mixture was molded into a cylinder with a diameter of30 mm and a height of 12 mm, and then pressed in a CIP.

This molding was placed on a rod of yttrium-stabilized ZrO₂ and set in atube furnace with a controllable firing atmosphere. The sample washeated for 3 hours to 1075° C. in an atmosphere of 0.1% O₂+99.9% Ar, andafter being held at this temperature for 1 hour was immediately cooledto 1005° C. over a period of 15 minutes. NdBa₂Cu₃O_(7−δ) seed crystal,which had been produced in advance by melt solidification process, wasplaced on the sample during this cooling. After this, the sample wascooled down to 950° C. at a rate of 0.5° C. per hour. The sample thusproduced was in the form of a single domain with a uniform crystalorientation.

The (Nd_(0.33)Eu_(0.33)Gd_(0.33)) Ba₂Cu₃O_(7−δ)-based superconductingoxide material bulk obtained above was cut into a 3×4×5 mm³ (a plane tobe joined was 4×5 mm²) rectangular parallelepiped such that the plane tobe joined was parallel to the (110) plane. The surface of this cutsample to be joined was polished to a mirror finish.

Preparation of Solder Material

Raw powders of Gd₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be 0.90GdBa₂Cu₃O_(7−δ)+0.10Gd₂BaCuO₅, and these weremixed for 3 hours in an automated mortar and pestle. The mixed powderwas molded in a uniaxial press, after which it was calcined for 24 hoursat 880° C. under atmospheric conditions. After this calcination, thesample was pulverized again and then calcined three more times by thesame method as above. Next, the calcined powder was sintered for 24hours at 965° C. in an atmosphere of 1% O₂+99% Ar. The sintering andpulverization were repeated until all of the raw powder became twophases of GdBa₂Cu₃O_(7−δ) and Gd₂BaCuO₅.

Platinum was added to the mixed powder composed of two phases in anamount of 0.5 mass % with respect to the total mass of the mixed powder,and this was mixed for another 3 hours in an automated mortar andpestle. After this mixing, the mixture was molded into a rectangularparallelepiped approximately 18×9×5 mm³ in size, and then pressed in aCIP.

This molding was sintered by heating for 10 hours in an atmosphere of 1%O₂+99% Ar and at 975° C., which is 10° C. lower than the 985° C.peritectic reaction temperature of the GdBa₂Cu₃O_(7−δ).

The sinter thus obtained was cut into a plate with a thickness ofapproximately 1 mm, and further polished to produce a spacer with athickness of 0.5 mm. The spacer was polished until both sides had amirror finish.

Joining

This spacer with a thickness of 0.5 mm was sandwiched between two cutbulks as shown in FIG. 1. The sample was heated for 3 hours to 975° C.in an atmosphere of 0.1% O₂+99.9% Ar, held at this temperature for 1hour, and then cooled to 925° C. at a rate of 0.5° C. per hour.

Evaluation

After the heat treatment, a sample was cut from the obtained joinedproduct so as to include the joined interface as shown in FIG. 2, partof this was polished in order to observe the microstructure at thejoined interface, and the other part was subjected to oxygen annealing,in which the temperature was raised to 450° C. over 3 hours in an oxygenatmosphere, then the temperature was lowered to 300° C., taking 50hours, and finally the temperature was held there for 150 hours, inorder to evaluate the superconductivity characteristics.

Texture images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed that the joined portion wasextremely dense, with no pores visible, indicating that the joiningprocess was successful. Furthermore, it can be seen that there was nosegregation of CuO or the like, indicating that the microstructure wasextremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. The superconductivity transition temperature of the samplewas 94 K. The critical current density was approximately 50,000 A/cm² atan external magnetic field of 2 T. The irreversible magnetic field wasapproximately 6.5 T.

Example 8 Joining a Y—Ba—Cu—O Bulk with an Er—Ba—Cu—O Solder Material

A joined product was produced in the same manner as in Example 1, exceptthat the molding CIP-treated and served as the solder material wasprocessed to a thickness of 0.5 mm without sintering, and the resultingmolding (compact) was used as a spacer.

An evaluation sample was produced from the resulting joined product andevaluated by the same procedure as in Example 1.

Texture images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed some pores in the joint, butthere was no segregation of CuO or the like, indicating that themicrostructure was extremely uniform.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. The superconductivity transition temperature of thissample was 92 K. The critical current density was approximately 7000A/cm² at an external magnetic field of 2 T. The irreversible magneticfield was approximately 3.5 T.

Comparative Example 3

Preparation of Matrix

A yttrium-based superconducting oxide material bulk in the form ofsingle domain oriented along the c axis (QMG made by Nippon SteelCorporation) was cut into a 3×4×5 mm³ (a plane to be joined was 4×5 mm²)rectangular parallelepiped such that the plane to be joined was parallelto the (100) plane. The cut sample was polished until both sides had amirror finish.

Preparation of Solder Material

Raw powders of Yb₂O₃, BaCO₃, and CuO were weighed out such that thecomposition would be YbBa₂Cu₃O_(7−δ), and these were mixed for 3 hoursin an automated mortar and pestle. The mixed powder was molded in auniaxial press, after which it was calcined three times for 24 hourseach time at 890° C., two times for 50 hours at 920° C., and one timefor 100 hours at 920° C. in a pure oxygen atmosphere. This calcinationwas repeated until all of the raw powder became a single phase ofYbBa₂Cu₃O_(7−δ).

The YbBa₂Cu₃O_(7−δ) powder was mixed in ethanol to produce a slurry.This slurry was centrifuged to adjust its concentration to about 70%.

Joining

The joining surfaces of the yttrium-based superconducting oxide materialbulks were coated with this slurry and sandwiched together as shown inFIG. 1. The sample was heated to 300° C. to completely remove thebinder. The sample was then heated for 3 hours up to 995° C., held atthis temperature for 1 hour, and then cooled down to 945° C. at a rateof 0.5° C. per hour.

Evaluation

After the heat treatment, a sample to be measured was produced from theobtained joined product by the same procedure as in Example 1.

Texture images and characteristic X-ray images (Ba-Mα line and Cu-Kαline) near the joined interface revealed numerous pores. A great deal ofsegregation of CuO was also seen.

Next, the superconductivity transition temperature and magnetic fielddependence of the critical current density were measured with a SQUIDmagnetometer. A magnetic field was applied parallel to the c axis of thesample being measured, and an external magnetic field of 10 Oe wasapplied in the measurement of the superconductivity transitiontemperature. The temperature was 77 K in the measurement of the magneticfield dependence of the critical current density. The superconductivitytransition temperature of this sample was 92 K. The critical currentdensity was approximately 50 A/cm² at an external magnetic field of 2 T.The irreversible magnetic field was approximately 2.9 T.

The compositions of the matrices and solder materials, the properties ofthe joined products, and so forth in the examples and comparativeexamples given above are compiled in Table 1 below.

TABLE 1 Compositions of matrices and joining materials and evaluationresults Joining Joining Matrix material interface Tc IrreversibleJoining Main Segre- (*1) Jc (*2) mag. field Comp. plane phase Form Poresgation (K) (A/cm²) (T) Ex. 1 Y123 (110) Er123 sinter no no 92 app. app.4.8 13,000 C. E. 1 Y123 (100) Er123 sinter no yes 92 app. app. 4.0 5000Ex. 2 Y123 (110) Yb123 sinter no no 92 app. app. 4.0 10,000 C. E. 2 Y123(100) Yb123 sinter no yes 92 app. app. 3.7 5000 Ex. 3 Y123 (110) Y123sinter no no 91 app. app. 5.0 12,000 Ex. 4 Sm123 (110) Gd123 sinter nono 94 app. app. 6.0 20,000 Ex. 5 Sm123 (110) Gd123 sinter no no 95 app.app. 6.0 25,000 Ex. 6 Nd123 (110) Sm123 sinter no no 95 app. app. 6.025,000 Ex. 7 (Nd,Eu, (110) Gd123 sinter no no 94 app. app. 6.5 Gd) 12350,000 Ex. 8 Y123 (110) Er123 compact a few no 92 app. app. 3.5 7000 C.E. 3 Y123 (100) Yb123 slurry many much 92 app. app. 2.9 50 *1 Tc:superconductivity transition temperature *2 Jc: critical current densityat external magnetic field 2T C.E.: Comparative Example

It is clear from the results in Table 1 above that when the plane to bejoined of the matrix was parallel to the (110) plane as in the presentinvention, the joined surfaces were more strongly coupled together and ahigher critical current density was obtained than when a plane parallelto the (100) plane was used for the joined plane as in the past.

It can also be seen there that a joined product with fewer pores in thejoining layer and a higher critical current density was obtained thanwhen a compact was used in a case in which a sinter was used as thesolder material.

In the above examples, a sinter or compact was used as the sheet-formsolder material, but using a melt-processed plate will yield the sameresults as when a sinter is used.

INDUSTRIAL APPLICABILITY

With the joining method of the present invention, pores and CuOsegregation can be greatly reduced in the joined portion, and it ispossible to produce a large superconductor with uniform crystalorientation, without sacrificing the superconductivity characteristicsof the superconductor including the joined portion. A joined oxidesuperconductor produced by the joining method of the present inventioncan be used as a material for magnetic shields, superconducting magneticload transport system and superconducting permanent magnets, so thepresent invention is extremely useful for industrial purposes.

1. A method for joining an RE123 oxide superconductor produced by a meltprocess, comprising: aligning a plane to be joined of an RE123 oxidesuperconductor so as to be parallel to the (110) crystallographic plane;interposing a solder material composed of an RE123 oxide superconductormaterial having a lower melting point than the RE123 oxidesuperconductor between the planes to be joined; and melting and thensolidifying said solder material to form a joining layer, wherein RE isone or more members selected from the group consisting of Y, La, Pr, Nd,Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu.
 2. The method of claim 1, whereinthe solder material is a sinter or a melt-processed plate.
 3. The methodof claim 2, wherein a surface of the solder material is finely polished.4. The method of claim 1, wherein the solder material is a powder, aslurry, or a molded powder.
 5. The method of claim 1, wherein the RE123oxide superconductor contains a non-superconducting phase including RE.6. The method of claim 5, wherein the non-superconducting phase is anRE₂BaCuO₅ phase as a RE211 phase, and/or an RE_(4−x)Ba_(2+x)Cu₂O_(10−y)phase, as a RE422 phase, wherein 0≦x≦0.2, 0≦y≦0.5.
 7. The method ofclaim 5, wherein the non-superconducting phase is an RE₂BaCuO₅ phase, asa RE211 phase, and/or an RE_(4−2x)Ba_(2+2x)Cu_(2−x)O_(10−y) phase, as aRE422 phase, wherein 0≦x≦0.3, 0≦y≦0.6.
 8. The method of claim 1, whereinthe RE123 oxide superconductor contains one or more members of the groupconsisting of Ag, Pt, CeO₂, and Ag₂O.
 9. The method of claim 1, whereinthe solder material contains one or more members of the group consistingof Ag, Pt, CeO₂, and Ag₂O.
 10. The method of claim 1, wherein a pressureis applied during joining.
 11. A joined RE123 oxide superconductorproduced by the method of claim 1, in which (110) planes of a pluralityof RE123 oxide superconductors produced by a melt process have beenjoined with a solder material composed of an RE123 oxide superconductormaterial having a lower melting point than the RE123 oxidesuperconductor to be joined, wherein RE is one or more members selectedfrom the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm,Yb and Lu.